1. Field of the Invention
The present invention relates generally to pattern inspection technologies and, more particularly, to pattern inspection techniques for inspecting a semiconductor fabrication-use workpiece pattern for defects. This invention also relates to an apparatus for inspecting lithography masks for defects, which are adaptable for use in the manufacture of semiconductor devices and liquid crystal display (LCD) panels.
2. Related Art
In recent years, with the quest for higher integration and larger capacity of large-scale integrated (LSI) circuits, semiconductor devices are becoming narrower in circuit linewidth required. These semiconductor devices are fabricated by using an original or “master” plate with a circuit pattern formed thereon (also called a photomask or a reticle as will be generically referred to as a mask hereinafter) in a way such that the pattern is exposure-transferred by reduced projection exposure equipment, known as a stepper, onto a target wafer to thereby form thereon a circuit. Hence, for the manufacture of a mask to be used to transfer such ultrafine circuit pattern onto wafers, pattern photolithography equipment is used, which is capable of “drawing” microcircuit patterns. Such pattern exposure equipment is also employable in some cases to directly draw or “image” a circuit pattern onto wafers. As for the pattern exposure equipment, an attempt is made to develop exposure tools using an electron beam or laser beam.
Improving manufacturing yields is inevitable for the microfabrication of LSI chips which entail increased production costs. Currently, circuit patterns of LSIs, such as 1-gigabit class dynamic random access memories (DRAMs), are becoming on the order of nanometers (nm), rather than submicron order. One major factor for reducing yields must be pattern defects of a mask as used when an ultrafine pattern is exposed and transferred onto semiconductor wafers by photolithography techniques. As LSI patterns to be formed on semiconductor wafers are further miniaturized in recent years, the size dimensions that must be detected as pattern defects became much smaller than ever before. Thus, a need is felt to achieve further increased accuracy of the pattern inspection apparatus operable to inspect the LSI fabrication-used pattern-transfer mask for defects.
Incidentally, with recent advances in multimedia technologies, LCD panels are becoming larger in substrate size, up to 500 mm×600 mm or more, and finer in pattern of thin film transistors (TFTs) as formed on liquid crystal substrates. This larger/finer trend requires an ability to inspect ultrasmall pattern defects in a wide range. For this reason, it is an urgent challenge to develop an advanced workpiece inspection apparatus capable of efficiently inspecting defects of photomasks in a short time period, which are for use in the manufacture of such large-area LCD patterns and large-screen LCD panels.
Here, in currently available pattern inspection tools, it is known to perform inspection by comparing the optical image of a pattern on a workpiece, such as a lithography mask or else, which image is sensed by using a magnifying optical system at a specified magnification, to either design data or a sensed optical image of an identical pattern on the workpiece. This approach is disclosed, for example, in JP-A-8-76359.
Examples of pattern inspection methodology include a “die to die” inspection method and a “die to database” inspection method. The die-to-die inspection is for comparing together optical images as sensed from identical pattern elements at different locations on the same mask. The die-to-database inspection is usually performed using an exposure device for drawing or “imaging” a pattern on a mask and an inspection device. Typically this inspection has the steps of receiving computer-aided design (CAD) data indicative of a designed pattern, converting the CAD data to pattern draw data having a format appropriate for data input to the imaging device, inputting the converted data to the inspection device, causing it to generate a reference image based thereon, receiving measured data indicative of the optical image of a pattern under testing as obtained by pickup of this pattern, and then comparing the optical image to the reference image to thereby inspect the under-test pattern for defects. The inspection method for use in such apparatus, the workpiece is mounted on a stage, which moves to permit light rays to scan a surface of the workpiece for execution of the intended inspection. A light source and its associated illumination optical lens assembly are used to emit and guide the light to fall onto the workpiece. The light that passed through the workpiece or reflected therefrom travels via the optics to enter a sensor so that a focussed optical image is formed thereon. This optical image is sensed by the sensor and then converted to electrical measurement data, which will be sent to a comparator circuit. After position-alignment between images, the comparator circuit compares the measured data to reference image data in accordance with an adequate algorithm. If these fail to be matched, then determine that pattern defects are present.
As previously stated, the quest for higher performance of semiconductor devices results in minimization of feature sizes and in increase in integration densities of chips. This trend in turn requires inspection equipment to offer higher resolutions. To this end, a need is felt to shorten the wavelength of illumination light. It is thus required to employ a laser light source having an inspection wavelength of deep ultraviolet (DUV) region. The currently available semiconductor road map suggests that for nodes of 90 nanometers (nm) or beyond, it becomes necessary to use illumination light with its inspection wavelength of 266 nm or less.
While it is desirable to use continuous wave light as the illumination light of inspection apparatus in order to permit a pattern image to accurately focus on a sensor, it is a must in order to obtain a continuous wave light source to perform wavelength conversion of a continuously oscillating long-wavelength laser beam for use as a fundamental wave to a wave having a shorter wavelength. More specifically, it is required to perform summed frequency generation while letting a plurality of continuous output laser beams of longer wavelengths than a prespecified wavelength being as the fundamental wave. However, such wavelength conversion is inherently a nonlinear process, and thus requires the use of high electric fields for enhancing the conversion efficiency. Additionally in view of the fact that the continuous oscillation essentially gives low electric fields, a special technique is needed for such wavelength conversion. Here, in order to increase the electric field intensity in a nonlinear medium, it is a must to employ a resonator structure of the type confining the fundamental wave in a nonlinear crystal. An example of the resonator used for sum frequency generation is an intra-cavity resonator having its laser amplifying medium as installed within the resonator. Another example is an extra-cavity resonator with a fundamental wave generation source being external to the resonator for the sum frequency generation use. An example of a continuous wave light source employing the resonator structure is disclosed, for example, in JP-A-10-341054, which is designed to emit 193-nm wavelength continuous wave light.
Unfortunately, the advantage of prior known continuous wave light source does not come without accompanying penalties of unwanted size increases. This can be said because these are required to use a large-size argon laser at part of a fundamental wave light source and also introduce resonators of the type stated above in order to emphasize fundamental wave electric fields. Another problem faced with the prior art lies in the risk of disturbance susceptibility as such resonators are relatively long in optical path for spatial propagation. A further problem is that the use of complicated configuration necessitates workers to consume much time to do maintenance services. Obviously a need for frequent maintenance works forces resonators to have a limited length of uninterrupted operation time period.
Another further problem encountered with the prior art resonator-associated light sources is that their complicated structures and increased size dimensions make it difficult to achieve installation into inspection equipment. If such installation is enabled, it still remains difficult to integrate the light source with the optics of inspection equipment. For these reasons, the light source is usually designed so that it is laid out at locations adjacent or next to its associative inspection apparatus or, alternatively, mounted spaced apart from the optics of inspection equipment. In this case, illumination light as given off from the light source is typically designed to travel along a long optical path prior to arriving at the optics of inspection apparatus, through spatial propagation with complicated combinations of mirrors and lenses. This poses a problem as to increased affectability of vibrations occurring due to motions of a wafer/mask support stage in the inspection apparatus. This in turn compels manufacturers to exert strenuous efforts for maintenance management of optical axes. Additionally, the difficulty in integrating the light source with the inspection apparatus badly behaves to limit designs of such inspection apparatus.
Another known approach is to employ as the inspection tool light source a short-wave length laser light source using a high electric field-obtainable pulsed laser beam. Even in this approach, it is still difficult to build the laser light source per se in the inspection equipment. Even when such is enabled, it is difficult to integrate it with the optics of inspection equipment. This brings similar results—i.e., the laser light source must be laterally disposed adjacent to the inspection equipment or spaced part from the optics thereof, resulting in output illumination light of the light source being forced to propagate along an elongate optical path prior to reaching the inspection equipment optics through spatial propagation with complexity of mirrors and lenses combined together, which causes a vibration influence problem similar to that stated supra. Thus, this laser light source also suffers from similar disadvantages to those stated above—i.e., the need for taking much time to do optical-axis maintenance management and limited design of inspection equipment.
Short-wavelength laser light sources using the high electric field-acquirable pulse laser light include a pulse light source with an ability to obtain a 193-nm pulsed laser beam by eight-harmonic generation from 1.5-micrometer light, examples of which are disclosed in JP-A-2001-83557 and the 23rd Annual Conference on Lasers and Electro-Optics (CLEO 2003) and the 11th Quantum Electronics and Laser Science Conference (QELS 2003), Paper No. CTuT4. An inspection tool with a built-in 199-nm light source is taught, for example, from the 24th Annual BACUS Symposium on Photomask Technology (September, 2004, Monterey, Calif., USA), Paper No. SPIE5567-110.
It has been stated that prior known light sources for obtaining the illumination light of pattern inspection apparatus are faced with various problems.